pH value-dependent electronic absorption and Ru(III/II) potential of bis-tridentate pincer ruthenium complexes

Add time:07/19/2019    Source:sciencedirect.com

Two bis-tridentate pincer complexes [Ru(H2bip)(Mebib)](PF6)2 (1(PF6)2) and [Ru(H2bip)(H2bib)](PF6)2 (2(PF6)2) were prepared and studied, where H2bip is 2,6-bis(benzimidazol-2-yl)pyridine, Mebib is the 2-deprotonated 1,3-bis(N-methylbenzimidazol-2-yl)benzene, and H2bib is 2-deprotonated 1,3-bis(benzimidazol-2-yl)benzene. These complexes were isolated as cyclometalated Ru(III) salts with a [(NCN)(NNN)] coordination due to a low Ru(III/II) redox potential. Complex 1(PF6)2 has two N−H protons on the NNN ligand side, while complex 2(PF6)2 has four N−H protons. Absorption spectral analysis showed that two deprotonation steps were distinguished for 1(PF6)2 with pKa1 and pKa2 of 3.46 and 5.88, respectively. In comparison, four deprotonation steps were observed for 2(PF6)2 with four pKa values of 3.45, 5.95, 8.95, and 10.98, respectively. When the solution was changed from acidic to basic, the Ru(III/II) redox wave of 1(PF6)2 and 2(PF6)2 shifted from +0.20 V vs Ag/AgCl to a less positive region by 540 and 850 mV, respectively. These studies suggest that the electronic absorption and electrochemical properties of these complexes are very sensitive to pH effect.

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